s390x/pv: Implement a CGS check helper

s390x/pv: Implement a CGS check helper

When a protected VM is started with the maximum number of CPUs (248),
the service call providing information on the CPUs requires more
buffer space than allocated and QEMU disgracefully aborts :

    LOADPARM=[........]
    Using virtio-blk.
    Using SCSI scheme.
    ...................................................................................
    qemu-system-s390x: KVM_S390_MEM_OP failed: Argument list too long

When protected virtualization is initialized, compute the maximum
number of vCPUs supported by the machine and return useful information
to the user before the machine starts in case of error.

Suggested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>
Message-Id: <20230116174607.2459498-2-clg@kaod.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>